1. Field of the Invention
This invention relates to processing systems and more particularly to processing systems performing multiple tasks while sharing the same physical resources.
2. Background Art
Two types of data processing systems are known in the art. One type has a single workstation with input/output devices that communicate with its own separate processor. The other type has a plurality of workstations that share and access the same processor.
A user of either of these systems is confronted with a problem of not being able to effectuate interrupt driven events at the user's command. For example, while a user is using the system for creating a report using an editor, the user may need to interrupt the word processing session and switch instantly to another program, such as to look up a phone number that resides in a program that is different from the one the user is using as an editor.
This problem has been addressed such that a system allows the output display to share its screen with a plurality of programs at the same time. To accomplish this, the screen is divided such that each program is shown on its own portion of the output display screen. This is commonly referred to as screen sharing.
However, this method of screen sharing is expensive since a large amount of CPU cycles are required. Screen sharing is expensive because all of the multiple windows contained on the one screen must be maintained. Also, there is an additional expense in clipping the output display of the application to subscreen boundaries.
Another problem with screen sharing is that a large amount of development time is needed to write the application program in a way that it will be executable in a screen sharing environment. Also, each application must be aware that it is working in this multiple activity environment.
Screen sharing also imposes additional requirements on an application. Usually the applications have to share resources such as the screen display. The applications must also agree to the amount of screen that each application will receive. Thus, the application must know that it is working with something less than the full portion of the screen.
Another problem arises with processing systems that utilize an UNIX* operating system. UNIX operating systems allow multiple activities, thus creating a multi-process system. This is referred to as a multi-thread environment. This allows a user to run several applications at one time. However, only one application can actually interact with the terminal. If more than one application tries to interact with the terminal at the same time, there is an undesired conglomeration of output from the applications. The user must then control the output from the other applications in the background. This is called blind background processing, which is difficult to monitor. FNT * UNIX is a trademark of AT&T Bell Laboratories.
One way to solve this multi-thread problem is to add physical devices to the workstation. However, it is not expedient for every task and/or virtual machine to have a unique physical terminal. The cost of multiple displays, keyboards, locators, and other interactive resources prohibits a profusion of such devices. A workstation is generally restricted to the number of devices it can support by the adaptor slot or the power limitations. The facilities such as office or desk space, electrical outlets, etc., also place constraints in the number of devices that a user can effectively use. There are additional inconveniencies in the physical movement of multiple devices. Also, refocusing of one's concentration is required when a multiplicity of interactive devices are used.